The long period fiber grating (LPFG) was fabricated by using the 800 nm femtosecond laser with line-by-line method in the non-hydrogen-loaded HI1060 fiber. The resonance wavelength of the LPFG is 1 548.4 nm, the resonance intensity is 12 dB, and the length of the grating region is less than 4 mm. The refractive index properties of LPFG were tested and studied by using different refractive index of NaCl solution, sucrose solution and alcohol solution. In the experiment, the resonance wavelength shifts toward longer wavelength with increases of the refractive index of three kinds of solution. The refractive index sensitivity of three kinds of solution is up to 175.34 nm/RIU, 175.31 nm/RIU and 331.89 nm/RIU respectively. The experimental results show that the LPFG based on the femtosecond laser has excellent sensitivity with the refractive index change of liquid, and can be used for the sensitive measurement of the liquid refractive index.